1. Field of the Invention
The invention relates generally to optical fibers. More specifically, the invention relates to a large mode area fiber employed as an optical filter and exhibiting a refractive index profile that minimizes bend-induced limitations on the fiber's filtering properties.
2. Discussion of the Related Art
In the field of optical fiber-based technology, there is an increasing interest in the use of large mode area fibers, including fiber-based optical filters, in which it is desirable to filter or suppress certain wavelengths of light. Such large mode area filter fibers are known to overcome various nonlinear impairments. For example, in some fiber amplifiers and lasers, stimulated Raman scattering (SRS) may limit power and efficiency. Suppressing wavelengths associated with SRS has been demonstrated to significantly improve the performance of such amplifiers and lasers. Some amplifiers may also require suppression of amplified spontaneous emission or spurious lasing at one wavelength (e.g. 1060 nm) in order to provide efficient amplification of a wavelength with competing or intrinsically lower gain (e.g. 940 nm). Similarly, significant benefits of wavelength filtering have been demonstrated in applications such as telecommunications, sensing, etc.
In large mode area filter fibers, there is typically a tradeoff between degree of filtering, effective area, and bend loss, e.g., a required degree of filtering becomes more difficult to achieve as mode area increases. Other properties (beam quality, pump absorption, birefringence, etc.) may also play an important role in overall performance, and limit the degree of filtering.
Bend perturbations play an important role in determining both the bend loss and the degree of filtering. For bend radii typically required for reasonable handling and packaging of conventional filter fibers in operation, the bend perturbation significantly degrades achievable performance.
When conventional filter fibers are employed in high power amplifiers and lasers, for example, overall system performance (output power, pulse energy, etc.) is limited by a failure to attain large effective area, low bend loss, and strong filtering when the filter fiber is arranged in a practical coil size.
At least two different approaches have been employed to minimize bend-induced losses in optical filter fibers. In one approach, a composite filter fiber is kept substantially straight by means of the incorporation of rod-like fibers that are extremely bend resistant. By forcing the filter fiber to remain essentially linear, bend-induced loss can be significantly reduced. This approach may be impractical for many applications, especially when fiber lengths are on the order of one meter or more. That is, this approach may achieve large area, low loss, and strong filtering, but fail to meet constraints on fiber arrangement needed for some applications.
A second approach is associated with predetermining a fixed bend loss by defining a specific “coiling” to be used (consistent with packaging and other practical constraints on fiber arrangement), and then utilizing the filter fiber in accordance with the specified coiling radius (and number of turns). This approach is subject to bend-induced limitations discussed above, for example the tradeoff between mode area, bend loss, and filtering. Similarly, this approach is considered to limit applications of large area filter fibers, as well as limit modifications in field implementations and variations in the use of filter fibers.
Thus, a need remains in the art for a large mode area filter fiber that simultaneously attains large effective area, low bend loss, and strong filtering when the fiber is arranged in a practical coil size.